This invention relates to the photolithography, more particularly, to a phase shifting mask for improving the resolution and the method of manufacturing the same.
According to the recent trend toward the high integration and denser package of semiconductor devices, it is required to transfer the fine line patterns delineated in the photomask to the wafer as is desired, so there have been proposed the particularly modified manufacturing techniques.
Generally, the photolithography is the technique that enables to transfer the delineated image pattern in the mask to the photoresist film on a substrate, the light such as ultraviolet being incident upon the photoresist film through the mask.
The general photomask has the delineated features of transparent and opaque portions, which makes it possible to be selectively exposed. However, the resolution is no longer improved by the diffraction resulting from the increased pattern densities.
There have been widely studied various techniques for improving the photolithography resolution by employing the phase shift lithography technique.
A phase shifting lithography uses the transparent portion comprising a general transparent portions and another transparent portions with 180 degrees phase shifting which is made of a phase shifting material. Thus it is possible to solve the diffraction as well by the interference between the transparent portions at the opaque portion.
Accordingly there are developed various lithography techniques, for example, the sharp modulation of the light intensity, which makes it possible to approximate the delineated features to the image patterns in the mask, and the transfer of very complicated patterns as well.
There are provided a Levenson type mask and an edge enhancement type mask, the former has a transparent optical phase shifting layer formed in either transparent portion or other transparent portion adjacent to that portion and, the latter shifts the phase of the light at ends of the different 2 transparent portions to reduce the light intensity after the propagation of the light through the mask.
In the fabricating of the edge enhancement type mask, a pattern part of phase shifting material is prepared such that the pattern part has the width larger than that of the light shielding layer underneath the pattern part and then both side walls of the light shielding layer are selectively etched away with the pattern part served as an etch mask. Therefore the effect of the phase shifting layer is exhibited depending upon the width of the light shielding layer.
FIGS. 1A through 3C are sectional views explaining the process steps of a phase shift mask according to conventional art.
On a transparent lower substrate 11 is formed an opaque light shielding layer 12 on which a phase shifting layer 13 is disposed (FIG. 1A).
With a photoresist layer 14 as a mask, the stacks of the phase shifting layer and light shielding layer is etched away to expose the surface of the transparent lower substrate 11 (FIG. 1B).
After removing the photoresist layer 14, an isotropic etching or wet etching is carried out to remove the end portions of the light shielding layer 12 masked by the phase shifting layer thereon so that the pattern width can be adjusted, thereby the required mask is manufactured (FIG. 1C).
FIG. 2 illustrates the light intensity distribution associated with the conventional phase shifting mask.
As seen from FIG. 2A showing the light intensity in wafer which is subjected to the optical radiation, an accurate phase shifting is not introduced in the end portions of the conventional phase shifting mask used, which results in the photoresist pattern 16 on the wafer 15 as shown in FIG. 2B.
This is because the wet etching produces the under cut configuration of the peripheral portions of the shielding layer which affects the accurate phase shifting effect.
Furthermore, when the phase shifting layer and light shielding layer is made to form their patterns, it can be shown the reverse critical dimension structure which makes it difficult the correction of the unexpectedly defected mask.
Further, there exists an instability of the side surface due to the uniformity of the surface level.